Why do people get depressed during winter? Odds are you've heard of seasonal affective disorder, or you've experienced it for yourself. Fittingly abbreviated "SAD," this periodic melancholy is most often seen in Northern latitudes with the long nights and short days of nature's coldest season.

But what's really going on when your body catches a case of wintertide doldrums?

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Winter-onset seasonal affective disorder was first described by researchers at the National Institute of Mental Healthin 1984, but people have been suffering from winter's depressive touch since long before the syndrome had a name. Nowhere is this more evident than the work of writers and poets, who throughout history have described winter as something to be endured, a season marked by perseverance in the face of soul-sapping chill and darkness.

"Every mile is two in winter," wrote 17C English poet George Herbert, pithily encapsulating winter's physical, psychological and emotional toll. The Mayo Clinic provides a more clinical description of SAD's symptoms, which include hopelessness, lethargy, social withdrawal, oversleeping and weight gain.

Of course, the hallmark symptom of SAD is that its effects tend to give ground during spring and summer, only to return on the heels of autumn. With this in mind, it's tempting to write off many of SAD's signature characteristics as emergent features of weather and culture. Winter, after all, can be a cruel, cold bitch; at what point does the desire to remain indoors, curled up beneath an electric blanket, come to constitute social withdrawal as opposed to, say, a completely rational preference for warm, dry conditions? Also, winter is the season of feasting; between Thanksgiving, New Year's and assorted holiday gatherings, doesn't everyone put on pretty serious wintertide poundage? (Actually, while answering the first question can be somewhat tricky, the widely held notion that the average person gains tons of weight during the holidays is a big, fat lie.) Perhaps not surprisingly, SAD was regarded skeptically by experts for many years — but more recently, several studies have helped validate the disorder.

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Most research identifies changes in daylength, or "photoperiod," as SAD's primary cause. Its commonness, for example, tends to vary by latitude. Epidemiological studies have shown that its prevalence in the adult population ranges from 1.4 percent in Florida to 10 percent in places like New Hampshire and Alaska.

What is important to keep in mind is that total amount of received light, its intensity and quality, do not matter in photoperiodic response in mammals. What matter is the duration of the night AS PERCEIVED BY THE SCN [aka the suprachiasmatic nucleus, the prime mover in the brain's regulation of circadian rhythms].

During the day, the SCN inhibits the secretion of melatonin. The duration of melatonin secretion is the signal for the duration of the night. This signal is then read and interpreted by other parts of the brain that trigger changes in development, morphology, physiology, reproduction and behavior in a seasonally appropriate manner. [Featured here: melatonin profiles in the blood of normal people in summer and winter, via Zivkovic's post]

But unlike animals, whose melatonin levels are dictated primarily by sunlight, humans are regularly exposed to artificial light. This can have a dramatic impact on our melatonin levels and, by extension, our physical and emotional wellbeing.

For instance, a recent study in the Journal of Applied Physiology, found that light emitted by your computer screen suppresses melatonin secretion and feelings of sleepiness, and has a significant impact on circadian physiology, alertness, and cognitive performance levels. Findings like these may explain why late-night computer users often have difficulty dozing off.

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But our neurochemical susceptibility to artificial light can be used to our benefit, as well. If computer light from a midnight Netflix-binge can knock your daily body rhythms out of sync, carefully timed exposure to bright light can set them right again. Such is the reasoning behind light therapy. For most people, light therapy involves exposure to bright light in the early morning. This helps curb the body's secretion of melatonin, reduces the brain's perceived duration of night, and returns the body to a more normal circadian phase.